Sycamore maple tree. Photo credit: Acer pseuoplatanus Chaltenbrunen by MurielBendel on Wikimedia Commons

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Many horse owners are confused about which trees are safe for horses and which are toxic to horses, especially when the trees have similar names. The sycamore maple is a prime example of confusing tree names. The trick to distinguishing which trees are related is to look at the scientific names. These are listed in the format of Genus, species.

American Sycamore

American sycamore leaves and seeds. Photo credit: American Sycamore (Platanus occidentalis) by Joseph OBrien, USDA Forest Service, via bugwood.org

The American sycamore is Platanus occidentalis. It is a tall shade tree with unique bark that peels off, revealing interesting patterns and colors. This tree is non-toxic to horses.

Red Maple

Red maple leaves before they turn red. Image credit: Red maple (Acer rubrum) by Paul Wray, Iowa State University, via bugwood.org

The genus name for maple trees is Acer. Red maple, Acer rubrum, is commonly known to be toxic to horses, primarily when wilted leaves are consumed by the horse. Its toxins cause destruction of red blood cells and prevent remaining red blood cells from transporting oxygen throughout the body. It is thought that the toxins in red maples may also be present in closely related maples (sugar maple, Acer saccharum; silver maple, Acer saccharinum) and hybrids of red maple. Red maple is native to North America and extremely abundant in the eastern United States.

London Planetree

London planetree leaves. Photo credit: London planetree (Platanus hybrida) by Robert Videko, Doronicum Kft., via bugwood.org

The naming confusion begins with the London planetree, or Platanus x acerifolia. From the genus Platanus, we know it is related to the American sycamore. The “acer” in acerifolia refers to the shape of the leaves—they are shaped like maple leaves, but are not related to maples. This tree is safe for horses.

Sycamore Maple

Sycamore Maple leaves and seeds. Photo credit: Sycamore maple (Acer pseudoplatanus) by Leslie J. Mehrhoff, University of Connecticut, bugwood.org.

Another confusing name is the sycamore maple. In Europe, where it is native, this tree is referred to simply as a sycamore. This tree is Acer pseudoplatanus, with Acer indicating that it is in the maple genus. The species name, pseudoplatanus, refers to its visual similarities to Platanus trees—hence, pseudo-platanus. This tree frequently causes poisoning in horses in Europe, where it is the most common maple species. Horses that consume the seeds or seedlings of this tree develop a highly fatal muscle disorder called atypical myopathy. A form of rhabdomyolysis, it is the result of toxins in the seeds and seedlings called hypoglycin A and MCPG. Levels of these toxins can vary widely among sycamore maple trees, and outbreaks tend to occur sporadically in certain years. It is considered an invasive species in some parts of the United States, including Pennsylvania.

Box Elder Maple

Boxelder leaves and seeds. Photo credit: Boxelder (Acer negundo) by Barbara Tokarska-Guzik, University of Silesia, via bugwood.org.

Another maple, the box elder maple (Acer negundo), is native to the U.S. and also contains hypoglycin A. Box elder maple seeds are responsible for a condition that closely resembles atypical myopathy called seasonal pasture myopathy. This serious condition is seen in the U.S. and usually develops in the autumn when seeds are falling. It can cause stiff gaits, incoordination, dark urine, rapid breathing, recumbency, and death. Seeds are only found on female box elder trees.

Conclusion

When selecting trees to plant in a pasture, always do some reading to determine whether the tree is toxic to horses. Make sure to use the tree’s scientific name and not its common name, as plants frequently share common names and may go by several different names depending on region.

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